Arthropods like this bee and most mollusks have open circulatory systems.įor more complex organisms, diffusion is not efficient for cycling gases, nutrients, and waste effectively through the body therefore, more complex circulatory systems evolved. Hemolymph returns to the blood vessel through openings called ostia. In (b) open circulatory systems, a fluid called hemolymph is pumped through a blood vessel that empties into the body cavity. Most vertebrates and some invertebrates, like this annelid earthworm, have a closed circulatory system. In (a) closed circulatory systems, the heart pumps blood through vessels that are separate from the interstitial fluid of the body. Birds, having a closed circulatory system, are thought to have moved more agilely, allowing them to get food faster and possibly to prey on the insects. In fact, one reason that insects with wing spans of up to two feet wide (70 cm) are not around today is probably because they were outcompeted by the arrival of birds 150 million years ago. An open circulatory system does not use as much energy as a closed system to operate or to maintain however, there is a trade-off with the amount of blood that can be moved to metabolically active organs and tissues that require high levels of oxygen. This movement allows for gas and nutrient exchange. As the heart beats and the animal moves, the hemolymph circulates around the organs within the body cavity and then reenters the hearts through openings called ostia. In an open circulatory system, the blood is not enclosed in the blood vessels but is pumped into a cavity called a hemocoel and is called hemolymph because the blood mixes with the interstitial fluid. As opposed to a closed system, arthropods-including insects, crustaceans, and most mollusks-have an open circulatory system, as illustrated in Figure 21.2 b. In a closed circulatory system, blood is contained inside blood vessels and circulates unidirectionally from the heart around the systemic circulatory route, then returns to the heart again, as illustrated in Figure 21.2 a. In all vertebrate organisms, as well as some invertebrates, this is a closed-loop system, in which the blood is not free in a cavity. The circulatory system is effectively a network of cylindrical vessels: the arteries, veins, and capillaries that emanate from a pump, the heart.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |